Warming, acidification, eutrophication, and the loss of oxygen are examples of major changes being observed or expected for the future in coastal zones around the world. These processes are occurring in the Baltic Sea at a much faster pace than in other regions. But the Baltic also provides useful lessons for how negative trends can be reversed by protective measures. In Science Advances, an international team of researchers led by the GEOMAR (Kiel, Germany) promotes the Baltic Sea as a time machine for coastal areas worldwide.